Why you have to modulate the signal?
Sometimes we have to send low frequency information, like the
tone of the voice of someone talking on a telephone, over a
specific channel that only permits a specific high frequency, such
as radio frequencies (RF). So how do we do that? One simple, cost
effective, and bandwidth efficient method is to change, or
modulate, a known RF signal a certain way. Some common ways of
modulating such a high frequency signal are Amplitude Modulation
and Frequency Modulation. Each of these modulation techniques has
its own advantages and disadvantages and are therefore more useful
in certain applications.